Coupling auxiliary cooling devices with ground source heat pump (GSHP) systems in cooling dominated areas can effectively solve the ground thermal imbalance problem. However, the input capacity of the auxiliary heat rejecter directly affects the performance of such hybrid ground source heat pump (HGSHP) systems. This paper presents an investigation on the optimal cooling tower input capacity of a cooling tower assisted GSHP system through both experiments and simulations. The experiments were carried out based on a HGSHP system implemented in an office building and the experimental results were used to validate a numerical model. A simulation system of the cooling tower assisted GSHP system was then developed using TRNSYS and validated against the experimental data collected. The impacts of the cooling tower input capacity on the soil temperature and the system performance were simulated. The results showed that the soil heat accumulation could be effectively alleviated when the cooling tower input capacity ratio (CTICR) was greater than 50%. The optimal cooling tower input capacity was highly dependent on the operation scenario used. The optimal CTICR under two operation scenarios considered was around 54%, while that under the other operation scenarios was around 63%. The control strategy based on the fixed temperature difference between the cooling water leaving the heat pump and the ambient air dry-bulb temperature was found to be the optimal control strategy for the system studied.